1. Field of the Invention
This invention relates to a system for managing heat fluxes of an aircraft.
2. Description of the Related Art
An aircraft comprises a cell and at least one propulsion system. In FIGS. 1 and 2, a cell was shown diagrammatically at 10 and a propulsion system was shown diagrammatically at 12.
According to a widely used embodiment, a propulsion system is suspended under a wing by means of a mast. More generally, the propulsion system is connected to the cell by means of an interface 14 that is embodied by dotted lines in FIGS. 1 and 2.
A propulsion system 12 comprises a turbomachine 16 that is equipped with a first engine cooling circuit 18, in which a coolant, in particular oil, circulates.
The turbomachine is supplied with fuel by means of a fuel circuit 20 that extends from a tank 22 that is arranged at the cell. As illustrated in FIG. 1, it is possible to use several cooling sources to cool the oil of the turbomachine, for example by using at least one oil/fuel exchanger 24 at the first engine cooling circuit 18, and to use the fuel as coolant so as to cool the oil of the turbomachine. In this case, a recirculation circuit 26 is provided so as to reintroduce the fuel that is heated in the tank 22. In addition, the propulsion system 12 can comprise another source 28 of thermal effluents, for example one or more electric generators installed close to the turbomachine.
So as to optimize the operation of these elements 28, it is necessary to regulate their temperatures by means of a second engine cooling circuit 30, in which a coolant that passes through a third engine exchanger 32, in particular an oil/fuel exchanger according to FIG. 1, circulates.
The characteristics of each engine cooling circuit, namely the characteristics of the fluid to be cooled, for example its flow rate, the characteristics of the exchanger, for example its dimensions, the characteristics of the fluid that is used for cooling, for example its flow rate, are adjusted based on the requirements for regulation of the temperature at the source, in particular so as to keep the temperature of the source below a certain threshold.
In the case of the first engine cooling circuit, these requirements vary according to the operation of the aircraft and are more significant when the aircraft is on the ground to the extent where the outside air can be at a high temperature and there is no air flow linked to the movement of the aircraft.
Thus, the characteristics of the engine cooling circuit relative to the turbomachine are generally determined based on the most significant constraints when the aircraft is on the ground.
The cell 10 also comprises at least one source of thermal effluents 34 and in general several sources 34, 34′, for example electrical accessories, electronic power systems, an air-conditioning system, avionics, and client equipment. All of these elements are to be temperature-regulated to ensure their operation and to guarantee the highest availability rate. For this purpose, at least one cell cooling circuit 36 is provided. According to the illustrated example, the cell comprises two cell cooling circuits 36, 36′, each comprising an exchanger 38, 38′ that makes it possible to cool the coolant that circulates in each of the circuits.
According to an embodiment that is illustrated in FIG. 1, the exchangers 38, 38′ are arranged in at least one cooling air channel 40 that is arranged in the lower portion of the fuselage and that comprises—upstream—one or more air intakes 42, preferably of the dynamic type, and—downstream—one or more exhausts 44.
The cooling air channels 40 operate according to two primary methods:                The first mode of operation takes place on the ground when the aircraft is immobile or moves at reduced speeds. In these cases, the natural air flow within said cooling air channels 40 is generally very low and is to be forced using in particular an electric fan 46 to allow the evacuation of the heat fluxes.        The second mode of operation takes place in flight when the aircraft moves at high speeds in a cold atmosphere. In this case, the dynamic pressure at the air intakes is significant, and the ambient temperature is relatively low; the effectiveness of the exchanges is very significant, so that it is necessary to limit the flow rate of air circulating in the cooling air channels to not over-cool the thermal effluent sources.        
In all of the cases, the air intakes are never blocked because the cooling air requirements always exist regardless of the mode of operation. To the extent that the air intakes always interfere with the aerodynamic flow, the cooling air channels prove to be detrimental in terms of aerodynamic drag for the aircraft and therefore in terms of energy consumption of the propulsion systems.
According to a first variant, the cooling air channels have set dimensions and are consequently simple, light and reliable. However, whereby their dimensions are calculated based on the most significant requirements, their impact on the aerodynamic drag, and therefore on the consumption of the aircraft, is significant during the high-speed flight phases, whereas the requirements are normally low for these flight phases.
According to another variant, the cooling air channels have a variable geometry to adapt their dimensions based on requirements, but in this case, the channels prove complex, heavy, and not very reliable.
As illustrated in FIG. 1, for the cooling systems that are implanted in propulsion systems, the fuel tanks 22 can constitute heat sinks. Actually, it is known to one skilled in the art that even when the fuel level is at its lowest, the tanks have intrinsic capacities for absorbing the thermal effluents.
So as to eliminate the drawbacks linked to the cooling air channels, according to another variant illustrated in FIG. 2, the exchangers 38, 38′ are not placed in a cooling air channel but ensure a heat transfer between the coolant of the cell cooling circuit(s) 36, 36′ and the fuel for conveying the heat fluxes in the direction of the tanks. For this purpose, at least one circuit 48 is provided between the tank(s) 22 and the exchanger(s) 38, 38′.
This relatively simple solution makes it possible to eliminate the cooling air channels and consequently is not detrimental in terms of aerodynamic drag and therefore energy consumption.
However, this solution is not completely satisfactory because its operating period is limited to the extent where it is no longer possible to dissipate the heat in the tanks when the fuel temperature reaches a certain threshold linked to the fuel temperature that is accepted by the turbomachines or to the risks of inflammability of the tank.
Consequently, when this threshold is reached, the capacities for heat dissipation are low, so that it is necessary to operate certain sources of thermal effluents in degraded mode; this is, for example, the case of the air-conditioning systems of the cell at the end of the flight.
According to other constraints, in terms of aircraft design, the components of the cell and the components of the propulsion systems are segregated for safety reasons. Actually, it is necessary to ensure that a breakdown that appears at the cell and disrupts the operation of the propulsion systems is extremely improbable.